The present invention relates to a single screw for polymer extrusion or injection-molding process, which provides enhanced melting and mixing performance.
Efforts aimed at improving melting and mixing action on single screw polymer extrusion and injection molding can be traced back as early as 1950's. The inherent shortcoming of single screw extrusion is that it is under the so called "laminar segregated melting mechanism". Compare to so-called "mix-melting mechanism" (examples of processes under later mechanism are internal batch mixer and twin-screw extruder), single-screw extrusion does not provide strong and uniform shear. It is this shortcoming which causes less efficient melting and poor mixing in compounding process. In the case of polymer-polymer or plastic-rubber blending processes, single-screw extrusion will not be able to provide the benefit of so called "phase inversion", a rate process discovered and defined by Chi-Kai Shih, DuPont Company, Inc., Wilmington, Del. (Ref. To "Plastics Engineering" June 1998, p.49).
Among the total five elementary steps (solid conveying, melting, mixing, pressurization and die forming), mixing and melting are the two slowest elementary steps. To get higher production and better quality, improved melting and mixing are a must. That is why tremendous efforts have been and will be invested into this field.
To get more efficient melting and mixing the key word here is shear, which is because:
A. shear energy can dissipate to heat energy (by "internal friction") thus melting the plastic; PA1 B. shear stress can break down the particle size of the filler (or in general the other phase), i.e., disperse mixing; PA1 C. shear strain or deformation, comes with shear stress, is essential to distributive mixing. PA1 (a) to provide intensive shear uniformly spread to the whole system by renewing the shearing surface constantly. The ununiform shearing or "laminar segregated melting mechanism" is so far the most reason why the single-screw extrusion was less efficient than twin-screw extrusion. Shear ring screw is going to make a change on this not preferred situation; PA1 (b) to provide intensive shear that allow the expose to high stress occurs only for a short period of time, which makes the process more efficient and less chance to get degradation. Further benefit of this feature is to avoid other problems could be caused by over shearing, such as re-agglomerate, chemical changes, molecule ramification branch changes, etc.; PA1 (c) to provide intensive shear and the strength of shear can be adjusted easily in a vast range. The reason we need shear level adjustable in a wide range is because too low a shear stress will not break agglomerate and too high will degrade the plastic (so called "burning"). A further reason is that different filler has different yield stress, so that a selectable range of shear strength is requested in order to get optimum result when filler or base resins changes. An example is a color concentrate manufacture, different pigment agglomerates need to be dispersed below certain critical sizes. PA1 (d) to provide intensive shear in the position you need along screw axis and that position can be changed easily. This feature is not usually found in prior arts. The benefit of this feature can be appreciated when switch one base material to another, one filler or additive to another, and one feeding option to another, etc.; PA1 (e) to provide intensive shear in a pattern that interrupt and re-organize flow stream lines vigorously, helping distributive mixing, which means homogenized extrudate. This benefit will appear more when a product demands dimension accuracy, such as sheet or profile extrusion, precision molding, etc.; PA1 (f) to provide complete barrel surface wiping action, which means self cleaning, no "dead corners"; PA1 (g) optional tube tip element offers surface renewal enhanced distributive mixing and stream kneading actions, which may especially benefit the Phase Inversion step and post Phase Inversion step, a rate process defined by Chi-Kai Shih, DuPont Company, Inc., Wilmington, Del. (Ref. To: ANTEC 1991 Conference proceedings, p99), in polymer blending process. The variations of the tip design could be a means of controlling droplet breakup/coalescence rates, extent of cross linking and in-situ grafting, etc, which are important to the properties of final products. It will also benefit in-barrel colorants, liquid additives, oils and tackifiers mixing, which are foreseeable booming markets as identified by Robert Barr, President of Robert Barr Inc., Virginia Beach, Va. (Ref. To: "Modern Plastics", July 1998, p79). PA1 (h) the design is simple, low cost on tooling, no special tools are required, easy to set up, which means short investment-profit cycle; PA1 (i) the design can be easily adopted to most popular extruders and injection molding machines, without changing barrel, die (or nozzle), heating, cooling, power supply, controlling system, etc . . .
Many patented and non-patented pin type, ring type, disc type, multiple-flight type, cavity type, barrier type, thin gap type and kneading type mixing screws or screw elements have been invented. Every type has it's own advantages and disadvantages. U.S. Pat. No. 4,652,138, to Inoue, etc. 1987, used two stage kneading portions, with the first stage disperses the filler into polymer by a strong shearing force, second stage blends and disperses the filler further by using the cavities in its rotor and stator. The so-called stator is actually a section of screw barrel which has cavities made inside of it. Although substantial improvement on mixing is expected, but undesired trapping of plastic melt in "dead spaces" may arise. Also its manufacture cost is high and can not be adopted to popular single screw barrel system.
U.S. Pat. No. 4,367,190, to Shirlay Beach, (1983), discloses using toothed rings as valve means on up stream and staggered arranged rows of pins on down stream to improve mixing on cable coating extrusion process. Similar design can be found in U.S. Pat. No. 4,015,833, to Heung Tai Kim, (1975), use shear rings with fins, and U.S. Pat. No. 4,103,353, to Timothy Stephen Dougherty, etc. (1976), using pins arranged in a ring pattern installed on certain sections of screw. The basic principle of above designs is to create tortuous path for plastic melt to go through thus enhance distributive mixing. Since these kind of designs have all of their mixing means fixed on the screw, their shear strength for mixing are limited.
U.S. Pat. No. 4,154,536, to N. Sokolow, (1979), discloses installing segmental mixing element to screw. This element is made of circumferentially interrupted helical flights. Further more, those screw flights has inclined ramps on the leading ends. This structure can increase solid bed shearing and conductive heating surface areas and thus enhance melting rate. In down stream barrier rings with advancing and reversing flights provide more shearing force to molten plastic, thus enhanced mixing. The weakness of this design is that, (a) It's not a easy job to make this screw, which means the tooling cost could be high; (b) Shear action may not uniform; (c) Too many broken flights may weaken the conveying strength too much, which means loosing on throughput.
Though a lot of efforts have been putting in, but "The truth is that there have been no major technology break through in screw design for last 30 years", indicated by a keynote speaker Robert Barr, President of Robert Barr Inc., Virginia Beach, Va., at ANTEC '98 Conference in Atlanta. He also envisions the next generation of screw designs as providing enhanced mixing and will be focussed on in-barrel melting and mixing (Ref. To: "Modern Plastics" July 1998, p.79). The shear ring screw will certainly help on above respects.